1. Field of the Invention
This invention relates to a thin-film magnetic head and more particularly to a thin-film magnetic head which has, on a substrate, a magnetic circuit consisting of upper and lower magnetic layers and a coil formed by a conductive layer and is provided with a magnetic shield layer.
2. Description of the Related Art
The thin-film magnetic head of the above-stated type is used for a magnetic disc device which is employed, for example, by an electronic camera and particularly for a magnetic disc device of the kind arranged to record and reproduce signals with a high degree of density. Spacing between recording tracks to be formed by the thin-film magnetic head on the magnetic disc device of this kind tends to become narrow because of a number of channels increased for shorter access time and also because of a demand for a higher degree of density. For example, the thin-film magnetic head employed in an electronic camera has been arranged to record one frame amount of a video signal in two channels in an analog manner. Whereas, in order to record a digital video signal or to perform high-definition image recording in a manner different from the conventional method, it is necessary to arrange a thin-film magnetic head to give a higher degree of track density and a greater number of channels.
FIGS. 1 and 2 show the arrangement of a typical multi-channel thin-film magnetic head, which is a three-channel head in this case. Referring to FIGS. 1 and 2, a substrate 1 is made of a non-magnetic material such as SiO.sub.2 or a ferromagnetic material such as ferrite. Thin-film magnetic head forming members are deposited on the substrate 1 in thin filmy states by photolithography.
A lower magnetic layer 2 is first provided in common for all channels over the whole upper surface of the substrate 1. The lower magnetic layer 2 is formed in the form of a thin film with a ferromagnetic material which is of a high-saturation magnetic flux density such as a Fe-Al-Si system alloy (called Sendust).
Coils 4 which are made of such a material as Cu or Al are disposed on the lower magnetic layer 2 through an insulation layer 7 which is a double layer. In this case, three coils 4 are arranged in parallel in the direction of track width for three channels 21 to 23.
An upper magnetic layer 3 is formed on each of the coils 4 through the insulation layer 7. These upper magnetic layers 3 are also made of a ferromagnetic material which is similar to the material of the lower magnetic layer 2. A magnetic circuit is formed by each of the upper magnetic layers 3 in conjunction with the lower magnetic layer 2 for a thin-film magnetic head. The upper magnetic layers 3 are arranged to straddle the respective coils 4. The fore end part of each of the upper magnetic layers 3 is opposed to the lower magnetic layer 2 via a magnetic gap 5. The rear end part of the upper magnetic layer 3 is brought into contact with the lower magnetic layer 2 at a contact part 6.
An insulation layer (not shown) is arranged to cover, for protection, the whole arrangement described above.
With the multi-channel thin-film magnetic head arranged in this manner, information is magnetically recorded or reproduced by causing the right-hand side of the head, as viewed on FIGS. 1 and 2, to come into sliding contact with the surface of a magnetic disc which is not shown. In recording, the magnetic disc is magnetized with a recording magnetic field generated at the magnetic gap 5 by allowing a recording signal current to flow to the coil 4. In reproducing, a magnetic flux is generated from a magnetized part of the magnetic disc located near the magnetic gap 5. The magnetic flux comes to be interlinked with the coil 4 passing through a magnetic path formed by the upper magnetic layer 3, the contact part 6 and the lower magnetic layer 2. A signal voltage is induced at the coil 4 as this magnetic flux varies according to the movement of the disc. The signal voltage which is thus induced is taken out as a reproduced signal.
In the above-stated multi-channel thin-film magnetic head, either the recording track width must be narrowed while leaving a spacing distance between the magnetic gaps 5 of the head as it is, or the distance between channels must be narrowed, in order to increase the recording track density. However, the narrowed track width does not give a sufficient reproduction output or makes tracking control difficult. Further, the narrowed distance between channels increases a crosstalk due to a magnetic flux leakage occurring between the channels.
The crosstalk problem is as follows: The magnetic flux leakage from a channel currently used for reproduction enters an adjoining channel to cause a noise by being interlinked with the coil. This problem must be solved in order to further increase the track density in the future.
Referring to FIGS. 3, 4 and 5 which show the flow of the magnetic flux, the generation of the crosstalk during reproduction is described in detail as follows:
FIG. 3 shows the flow of the magnetic flux of a first channel 21 which is assumed to be currently under a signal reproducing operation. A magnetic flux 49 which is generated from the magnetic recording medium enters the head from the sliding surface of the upper magnetic layer 3. The magnetic flux 49 comes to be interlinked with the coil 4 through a magnetic path formed by the upper magnetic layer 3, the contact part 6 and the lower magnetic layer 2. At that time, a part of the magnetic flux 49 passing the upper magnetic layer 3 leaks and comes into the upper magnetic layer of the adjoining second channel 22.
FIG. 4 shows the lower magnetic layer 2 as viewed from above. The position of each upper magnetic layer 3 is indicated by a broken line. Each contact part 6 is indicated by hatching. A magnetic flux 51 which comes from the upper magnetic layer 3 and the contact part 6 of the first channel 21 which is currently in the reproducing operation spreads to a great degree within the lower magnetic layer 2 as indicated by arrows before it comes back to the magnetic recording medium. Then, a part of the spread magnetic flux 51 flows to the lower magnetic layer 2 of the adjoining second channel 22.
FIG. 5 shows the magnetic flux flowing to the adjoining second channel 22. A magnetic flux 50 which leaks from the upper magnetic layer 3 of the first channel 21 and comes to intrude into the upper magnetic layer 3 of the adjoining second channel 22 forks into two flows near the magnetic gap 5 of the upper magnetic layer 3. One magnetic flux leak flow comes to the magnetic gap 5 and the other to the contact part 6. The magnetic flux 50 flowing to the contact part 6 comes back to the magnetic recording medium via the lower magnetic layer 2. Further, the magnetic flux 51 which comes from the first channel 21 and passes through the lower magnetic layer 2 spreads once toward the upper magnetic layer 3 at the contact part 6 and, after that, comes back to the magnetic recording medium through the lower magnetic layer 2. As a result of these magnetic flux flows, the part of the magnetic flux 50 which has intruded from the upper magnetic layer 3 and passed through the contact part 6 is interlinked with the coil 4 to induce a voltage. This brings about a crosstalk.
To lessen the crosstalk which is generated in the above-stated manner, practice has been to reduce the intruding magnetic flux leak by shortening the length of the upper magnetic layers 3 of the adjacent channels in such a way as to lessen their confronting areas.
This method, however, makes a coil winding space for the coil 4 too small. The coil must have a certain amount of sectional area for enduring a recording current. Hence, the narrow coil winding space results in a less number of turns of the coil which hardly gives a sufficient reproduction output. Further, coil winding within the narrow space makes the manufacture of the head difficult and thus results in a poor yield of production.
Another solution of the crosstalk problem has been proposed. In accordance with that solution, a slit is provided in the lower magnetic layer 2 between adjacent channels. The slit thus divides the lower magnetic layer 2, so that the magnetic flux leak from one channel can be prevented from intruding into another.
That solution, however, necessitates a photolithographic etching process on the lower magnetic layer 2 which measures several .mu.m to scores of .mu.m in thickness. The etching process is difficult and takes time. Further, in this instance, the crosstalk depends greatly on the width of the slit. The crosstalk characteristic thus tends to vary among products. Besides, the crosstalk characteristic has not been much improved by that method for all the manufacturing difficulty and high cost.
In a conceivable solution of the crosstalk problem, the thin-film magnetic head is arranged as follows: Referring to FIG. 6, a magnetic shield layer 9 which is made of an electrically conductive material such as Cu or Al is arranged to cover each of the upper magnetic layers 3 through a non-magnetic insulation layer which is not shown. With the head arranged in this manner, when the magnetic flux flowing to the upper magnetic layer 3 of a channel 21 increases to bring about a leak magnetic flux 50, the leak magnetic flux 50 tries to intrude into the upper magnetic layer 3 of the adjoining channel 22 as indicated by a broken-line arrow. Then, eddy currents 52 flow within the magnetic shield layer 9 as indicated by full-line arrows. The amount of the leak magnetic flux 50 intruding into the upper magnetic layer 3 is decreased by virtue of the eddy current 52, so that the crosstalk can be lessened.
In actuality, however, the crosstalk reducing effect of this method has been insufficient. The reason for this is as described below with reference to FIG. 7:
Referring to FIG. 7, the magnetic flux 50 which comes to intrude into the upper magnetic layer 3 from the adjoining channel is decreased by the magnetic shield layer 9 as stated above. However, the decrease causes another magnetic flux 51 which comes through the lower magnetic layer 2 to spread upward at the contact part 6. As a result, a part of the magnetic flux 51 comes back to the magnetic recording medium through the upper magnetic layer 3 and the magnetic gap 5 along with the magnetic flux 50. The flow causes the magnetic flux 51 to be interlinked with the coil 4. This results in a crosstalk.